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The Resonances of Loudspeaker Diaphragms

The resonance frequencies of several shapes of loudspeaker diaphragms have been calculated using BOSOR 4 (based on the Variational Finite Difference technique) and a Finite-Element programme. Calculations for a typical paper cone confirm that resonances are present over the whole frequency range, the lowest one being subsonic. The effect of altering the cone profile is to raise some frequencies and lower others. It is confirmed that sandwich construction can increase the dynamic bending stiffness by as much as several thousand times. Calculations on dome diaphragms suggest that with suitable design, there is a region where all resonances can be placed below or above the working range. The presence of a previously unsuspected torsional resonance is revealed by the method of calculation used. This resonance will not affect the frequency response and will be undetectable by holography, although it will affect the transient response. The method of calculation also enables buckling loads to be determined, so that it may be possible to calculate the onset of subharmonic.